1. Field of the Invention
This invention relates to a gas current classifying separator which is used for powder classification by causing the powder fed into a classification chamber to enter a high speed whirling vortex to be separated by centrifugation into a fine powder group and a coarse powder group (or medium powder group).
2. Related Background Art
When the powder starting material flowing into a classification chamber is fluidized in a whirl in said classification chamber, centrifugal force and air resistance force in the inward direction act on the respective particles of the powdery starting material, and the classification point is determined by the balance between the centrifugal force and the air resistance force.
At the outer periphery of the classification chamber, larger particles are whirled, while smaller particles whirl inside thereof. By providing powder discharging outlets respectively at the center and the outer periphery of the lower portion of the classifying chamber, the fine powder group and the coarse powder group can be collected separately (classification).
In such a classifying separator, it is important that the starting powder should be sufficiently dispersed within the classifying chamber to become primary particles in enhancing the classification precision.
As this kind of classifying separator, an Iitani system classifying separator or Kuracyclon has been proposed. However, in this type of classifying separator, it is very difficult to control the classification point, to and involves such problems such as poor dispersion and poor classification precision when there is high dust concentration. In order to solve such problems, various proposals have been made. For example, such proposals are disclosed in Japanese Patent Laid-open Applications Nos. 54-48378, 54-79870 or U.S. Pat. No. 4,221,655. As a classifying separator practically applied, there may be mentioned a commercially available classifying separator sold under the name of DS separator. In this kind of classifying separator, although it has become possible to control the classification point, since powder is fed through a cyclon section into the classifying chamber, the powder is concentrated before entering the classifying chamber, whereby dispersion of the powder tended to become insufficient. Accordingly, a low classification efficiency results. Referring now to FIG. 5 and FIG. 6 in the accompanying drawings, the prior art device is to be further explained.
FIG. 5 is a schematic view of the outer surface of the prior art device, and FIG. 6 a schematic sectional view of the prior art device.
In FIG. 5 and FIG. 6, the gas current classifying separator has a main casing 1, a lower casing 2 connected to the lower portion of said casing 1, and a hopper 3 at the lower portion of the lower casing 2. Internally of the main body casing 1 is formed a classification chamber 4. At the upper portion of the main body casing 1 stands a guide cylinder 10, and a feeding cylinder 9 is connected to the upper outer peripheral portion of said guide cylinder 10. At the bottom within the guide cylinder 10 is equipped a cone-shaped (umbrella-shaped) discharging guide plate 15 with a high central portion, and an annular inlet 11 is formed at the lower brim outer periphery of said discharging guide plate 15. At the bottom of the classifying chamber 4 is equipped a cone-shaped (umbrella-shaped) classifying plate 5 with a high central portion, and an annular coarse powder discharging outlet 6 is formed at the lower brim outer periphery of the classifying plate 5, and a fine powder discharging outlet 7 is formed at the central portion of the classifying plate 5. At the outer periphery of the lower surrounding wall of the classifying chamber 4, there is a gas inflow inlet 8 equipped for inflowing air. The air inflow inlet 8 is constituted generally of gaps between a plural number of blade-shaped louvers 14 (see FIGS. 15A and 15B). The direction of the air introduced through the gas inflow inlet 8 is controlled by the classification louvers 14 so as to be jetted out in the whirling direction of the powder material which descends under whirling in the classifying chamber 4. Said air disperses the powder material, and also accelerates the whirling speed of the powder material.
FIG. 4B shows a cross sectional view seen along III--III in FIG. 5 and FIG. 6. In such gas current classifying separator, the starting powder pressure delivered by gas current from the feeding cylinder 9 to the guide cylinder 10 descends whirling around the internal outer periphery of the guide cylinder 10 and flows whirling through the annular feeding inlet 11 into the classifying chamber 4. Within the classifying chamber 4, the powder is separated into a coarse powder group and a fine powder group through the centrifugal force acting on the respective particles. However, in the device of the prior art, since the starting powder is fed into the classifying chamber 4 while being concentrated at the inner wall of the guide cylinder, dispersion of the powder particles is insufficient, and the powder descends while drawing a spiral in band within the guide cylinder similar to a cyclone. Therefore a nonuniform concentration is fed into the classifying chamber, whereby it is difficult to obtain sufficient classification precision. When the fine powder forms an agglomerate, or when fine powder is attached to coarse powder, if dispersion is insufficient, fine powder increasingly tends to be mixed into the coarse powder group side. Further, if dispersion is insufficient, the dust concentration within the classifying chamber 4 becomes nonuniform, whereby the classification precision itself is worsened, thereby causing a problem that the classified product has a broad particle size distribution. This tendency is more marked as the particle size of the starting powder is finer. Particularly, when the powder is 10 .mu.m or less, the classification precision is lowered.
Accordingly, as disclosed in Japanese Utility Model Laid-open Application No. 54-122477, it has been proposed to prevent mixing of the coarse powder with the fine powder discharged through the fine powder discharging outlet 7 to make the average particle size of fine powder smaller by enlarging the diameter of the guide plate, enlarging the diameter of the feeding inlet and elongating the distance to the fine powder discharging outlet 7.
However, also in such a classifying separator, dispersion of powdery material within the classifying chamber is insufficient, and agglomerates of fine powder tend to be mixed into coarse powder, whereby lowering in classification efficiency departs from the first object of increasing the treated amount.